The present invention relates generally to tire retreading, and more particularly to improved methods and apparatus for curing envelopes for use in tire retreading.
In retreading a tire, the used casing is usually prepared by buffing off the worn tread to a profile at the periphery suitable for mounting a new tread. A pre-cured and buffed tread is then adhered about the periphery of the casing with a lamina of uncured cushion gum cemented between the tread and casing. A resilient outer curing envelope of molded synthetic rubber, such as manufactured by Presti Rubber Products of New Britain, Pennsylvania, the assignee of the present application, is placed around the periphery of the assembly and suitably sealed around the opposite outer walls of the casing or near the tire bead, and a vacuum is drawn through a valve in the envelope for stretching the envelope into intimate contact with all surfaces of the tread. The curing envelope thus assures during the curing process that no air or steam penetrates into the cushion gum, and that pressure is uniformly applied over the entire bonding area without tread distortion. The tire is then placed in the autoclave for several hours at an elevated temperature and pressure to cure the gum and positively bond the casing and tread together. The envelope is then removed for re-use.
Various techniques for sealing the envelope around the tire have been employed which require special mountings, rims and curing innertubes. However, in addition to increasing labor and equipment costs, such techniques often diminish the efficiency and uniformity of heat transfer for curing the cushion gum. Furthermore, if the tire casing has a small crack or hole, air may seep through and cause separation at the tread and casing interface. Even if a repair were applied at the hole, the environment of the autoclave may cause the repair to fail during the curing process if the repair is left unprotected.
An alternative arrangement for sealing the outer envelope uses a synthetic rubber inner envelope in combination with the outer envelope. One such inner envelope has been fabricated from four annular sections of calendered sheet seamed together to approximate the general cross sectional profile of the casing for covering the entire inside wall, bead, and annular inner portions of the opposite outer walls. An annular boss is cemented along the outer perimeter on which the outer envelope overlaps to form annular seals. Thus, when a vacuum is drawn, the space at the interface of the tire and the outer and inner envelopes is completely evacuated.
While such inner envelopes may function satisfactorily for some purposes, breaks in the many seams and cemented surfaces with frequent re-use prevent drawing the vacuum required between the tire and envelope interfaces. In addition, the relatively high pliability of the calendered sheet envelopes precludes simultaneously vulcanizing rubber on restored bead surfaces of the tire while retreading because the envelopes cannot retain the cross sectional profile of the bead during the curing process. Furthermore, there is a demand for form-fit durable inner curing envelope which can be readily installed on a tire in combination with an outer curing envelope, and re-used over a relatively long period of time before wearing out.